Everybody knows the standard procedure to keep your app alive, after the user pressed the lock button (silent sound). If I start a sound with AVAudioPlayer (before the iphone is locked), the sound plays till it's end (after locking). The app is still running. If I try to start another sound while the iPhone is locked, it will never get played. All the other things work as well but the sound doesn't.
How can I play a sound while the iphone is locked?
If the problem is that your app is doing to sleep after the screen locks, then this question is already answered here and here on SO